In a user equipment (UE) with multiple subscriber identity modules (SIMs), conflict occurs when different connections request access to the same radio frequency (RF) hardware, especially, for example, if the UE only has one transmitter. Such conflicts are resolved by a controller which uses a prioritization of data content within the different signals of the transmission time intervals (TTIs) requesting access to the RF hardware. As a result, data is discarded for one of the SIMs because of the limited access to the UE's available RF hardware.
Modern RATs like LTE have implemented a retransmission protocol for data packets on the physical (PHY) layer. This PHY layer retransmission makes the radio link friendlier to the transmission control protocol (TCP) as it decreases the variation in round trip time for the data. In order to ensure that a data packet is not stuck in an inefficient retransmission loop, e.g. in a sudden decrease in data quality, the data will only be retransmitted within a certain timeframe before expire. In uplink, the time between the transmissions is set at a fixed interval of 8 ms, whereas in downlink it is flexible and varies depending on the scheduler.
Multi-SIM operation may significantly increase the probability of momentary high round trip times for TCP data as it may discard packets which are close to expiration. This will lead to the system discarding data, which will trigger a slow start algorithm of the TCP.
Resolving a conflict within a Multi-SIM UE is currently done using a lookup table in which the SIM with the highest priority is granted access to the RF unit. However, as the amount of simultaneously supported Radio Access Technologies (RATs) increases, these lookup tables prove insufficient.